Originally Posted by
106459
There are two different technologies used on how cell phones work: GSM and CDMA. The major difference is that GSM uses SIM cards and CDMA does not - the technologies are incompatible with one another.
In the US these two technologies compete, and there is AT&T (GSM) vs. Verizon (CDMA), with the "discount" competition being Tmobile vs. Sprint (GSM v CDMA).
Unlocking a phone is entirely different, that's when a phone is purchased through a carrier and is locked to only their network (common when phones were subsidized and under contract). Nowadays I believe most phones are purchased at full price, I'm not sure how many come unlocked by default, or if you have to call the carrier and have it done -- I buy very few phones (1 smartphone total, haha), but unlocking shouldn't be a very big deal.
Also to note, there are very few phones that are manufactured with both technologies, but they do exist (newer iPhones and the Nexus 5, 6).
To answer the other questions -- you can buy any SIM card and they work on SIM card phones (it's all GSM). Not to confuse things, there are different sizes of SIM cards (which the carrier could immediately look up/give the proper size), its only affect is the size of the card/fitting in the phone itself. However, it sounds like this doesn't apply to your phone since it's probably CDMA (no SIM card).
--I don't know the process for switching CDMA carriers, it's not an unheard of thing and I'm sure it's relatively simple, involving taking your phone into a shop and having them set it up. Just check the phone to see if it's branded to a specific carrier (Verizon/Sprint) on it (probably needs unlocking), and ensure it's unlocked if needed.
On a last note, the only way to send a text/SMS message over a carrier network would be to have carrier service (irrespective of WiFi). Presumably, he's using one of the messenger apps (WhatsApp), etc, to send messages over their platform/app which only requires a data/wifi connection.
Last, last note: Your choice in carriers may be more constrained than you think - we tried using tmobile in my house, but while service was good on the edge of the neighborhood/outside, it sucked for us in our home which just doesn't really work. Ended up using AT&T.
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